I rejected the poison swamp immediately and didn't feel like dealing with the truck-sized crabs so I knocked off an invader and called it quits for the night.
On my first playthrough, I got almost to the Sage but didn't fight her. I got caught up with those two exiled nights living on the steps leading to Farron Keep, and they stomped me over and over until I moved to a different game. This time around I stomped them without taking a hit. Experience matters I guess.
I wasn't remotely ready to face a boss when I first stumbled into the fight area. I looked at it and thought "I bet this place has a bonfire right before the boss" and then immediately found myself facing a boss.
The upside of the poison area is that it has some good upgrades. Flask stuff, etc. There is also a bonfire near 3 mobs -- 2 spear guys and a magic user (that barfs poison clouds). It exists next to a bridge. That bonfire is great for leveling. Just kill the 3 mobs. Rest. Kill again, etc. Particularly good if you feel under-leveled OR have a stat you are close to getting to say, 30. Or if you need just 1 more 1000 souls later in the game for any reason (close to an upgrade cost, etc). It gives about 1k souls per reset. Beware the soft and hard stat caps though, so you maximize your points. I'm particular, so if I am at 29 strength and I can get 30 by farming that spot, I'll do so for the round #. In addition to all the upgrades in the area, there is a ladder that takes up you to a great spot with more upgrades, a mini boss, and even more upgrades, with a nice spot for praising the sun.
Those knights are hard -- 1st time I faced them I got sick of drinking estus so just went down the ladder. Found the bonfire. They followed me! Now I make short work of them, but experience is the huge factor like you said.
My favorite fights are when you don't realize once is coming (like CB and Father G in Bloodborne). Over time I've gotten a lot better at recognizing arenas though. I still like the "Surprise, MF!" moment. Or bosses that have multiple phases / tricks in store.
Edit: Give BB a chance after DS3 -- it really is a masterpiece. Once it "clicks" (usually at Vicar Amelia-- ) -- you're beyond hooked.